The field of the invention is magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to contrast-enhanced MRI methods and systems.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Magnetic resonance imaging (MRI) provides images with excellent contrast between soft tissues. To further enhance this image contrast, contrast agents are typically employed. One example where contrast agents have found wide use in the field of MRI is in contrast-enhanced (CE) magnetic resonance angiography (MRA), which uses the magnetic resonance phenomenon to produce images of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent, such as a gadolinium-based contrast agent, is injected into the patient prior to the MRA scan. The paramagnetic nature of gadolinium results in a decrease in the longitudinal relaxation time, T1, of protons in proximity to the contrast agent. The decrease in T1 is then manifested as an increase in signal intensity in a T1-weighted image.
Another example of where contrast agents have found use in MRI is in perfusion imaging. As described, for example, by K. A. Kemp, et al., in “Quantification of Regional Cerebral Blood Flow and Volume with Dynamic Susceptibility Contrast Enhanced MR Imaging,” Radiology, 1994; 193:637-641, it is possible to assess regional cerebral hemodynamics by analyzing MR signal intensity changes after the first pass of a paramagnetic contrast agent. While passing through the capillary network, a short bolus of the contrast agent produces local magnetic field inhomogeneities that lead to a reduction in the transverse magnetization relaxation time, T2*, of the bulk tissue. This susceptibility effect is recorded by a series of rapidly acquired T2*-weighted gradient-echo images that reveal how the MR signal changes during the first pass of the contrast agent. From this series of contrast-enhanced MR images, hemodynamic parameters such as blood flow, blood volume and mean transit time may be computed.